(a) Field of the Invention
The present invention relates to halogen-free flameproof thermoplastic molding compounds which comprise an aromatic polycarbonate, a styrene-containing copolymer, a styrene-containing graft polymer, a phosphorus compound, a silicon compound, and a tetrafluoroethylene polymer.
(b) Description of the Related Art
Flameproof polycarbonate/styrenic resin molding materials are widely used for electrical appliances. Most known flameproof blends of polycarbonate and styrenic resin contain halogenated flame retardant, which results in the release of toxic gas in the event of fire. To overcome such safety problems, halogen-free flameproof polycarbonate/styrenic resin molding materials have been developed by using phosphorus compounds.
U.S. Pat. No. 4,692,488 discloses thermoplastic molding materials containing a halogen-free aromatic polycarbonate, a styrene-containing copolymer, a phosphorus compound, and tetrafluoroethylene polymer as a coagulated mixture of styrene-containing graft polymers. The molding materials described in the U.S. Pat. No. 4,692,488 have satisfactory flame retardancy, but have completely inadequate impact strengths.
To increase the impact strength of the polycarbonate molding compositions which contain phosphorus compounds as a flame retardant, styrene-containing graft polymers have been used in prior art.
For example, U.S. Pat. No. 5,061,745 discloses polymer blends prepared from an aromatic polycarbonate, acrylonitrile-butadiene-styrene (ABS) graft polymer and/or styrene-containing copolymer, a monophosphorus compound, and a tetrafluoroethylene polymer. The patent shows that the use of ABS graft polymer increases impact strength.
U.S. Pat. No. 4,914,144 discloses polymer mixtures which consist of an aromatic polycarbonate, styrene-containing graft polymer based on an acrylate rubber, a styrene-containing copolymer, a monophosphorus compound, and a tetrafluoroethylene polymer.
U.S. Pat. No. 5,204,394 discloses polymer mixtures comprising an aromatic polycarbonate, a styrene-containing copolymer and/or styrene-containing graft polymer, and an oligomeric phosphate or a blend of oligomeric phosphates as flame retardant.
Although the above listed prior technologies have an adequate flame retardancy and mechanical properties, they have problems in achieving sufficient stress crack resistance.
When molding materials have poor stress crack resistance, problems such as cracks in molded parts may occur. Good stress crack resistance of the molding material is particularly important when the molding materials are injection molded into thin-walled parts or subjected to contact with chemicals.